Method for producing porous bodies with enhanced properties

ABSTRACT

A precursor mixture for producing a porous body, wherein the precursor mixture comprises: (i) milled alpha alumina powder having a particle size of 0.1 to 6 microns, (ii) boehmite powder that functions as a binder of the alpha alumina powders, and (iii) burnout materials having a particle sizes of 1-10 microns. In some embodiments, an unmilled alpha alumina powder having a particle size of 10 to 100 microns is also included in said precursor mixture. Also described herein is a method for producing a porous body in which the above-described precursor mixture is formed to a given shape, and subjected to a heat treatment step in which the formed shape is sintered to produce the porous body.

CROSS REFERENCE TO RELATED APPLICATIONS

The present invention claims the benefit of U.S. Provisional PatentApplication Nos. 62/169,706 and 62/169,766 filed Jun. 2, 2015, theentire content and disclosure of each are incorporated herein byreference.

FIELD OF THE INVENTION

The present invention relates to porous bodies and more particularly toporous bodies that can be used in a wide variety of applicationsincluding, for example, as a filter, a membrane, or a catalyst carrier.

BACKGROUND

In the chemical industry and the chemical engineering industry, relianceis oftentimes made on using porous bodies, including porous ceramicbodies that are capable of performing or facilitating separations orreactions and/or providing areas for such separations and reactions totake place. Examples of separations or reactions include: filtration ofgases and liquids, adsorption, reverse osmosis, dialysis,ultrafiltration, or heterogeneous catalysis. Although the desiredphysical and chemical properties of such porous bodies vary depending onthe particular application, there are certain properties that aregenerally desirable in such porous bodies regardless of the finalapplication in which they will be utilized.

For example, porous bodies may be substantially inert so that the porousbodies themselves do not participate in the separations or reactionstaking place around, on or through them in a way that is undesired,unintended, or detrimental. In applications where it is desired to havethe components that are being reacted or separated pass through, ordiffuse into, the porous body, a low diffusion resistance (e.g., higheffective diffusivity) would be advantageous.

In some applications, the porous bodies are provided within a reactionor separation space, and so they are desirably of high pore volumeand/or high surface area, in order to improve the loading and dispersionof the desired reactants, and also to provide enhanced surface area onwhich the reactions or separations can take place. These applicationsalso require sufficient mechanical integrity to avoid being damaged,i.e., crushed, chipped or cracked, during transport or placement.However, combination of high mechanical strength with high pore volumein a porous body is not easy to achieve because the strength decreasesexponentially with increasing porosity.

In view of the above, there is a need for providing porous bodies thathave a pore architecture that has enhanced fluid transport properties,particularly gas diffusion properties and high mechanical integrity.Such pore architectures can be achieved only by the precise control ofthe porous body precursor mixture and the porous body preparationprocess, which are described in the present invention.

SUMMARY

In an effort to reduce the level of defects in a porous body, theinstant disclosure is directed to methods for producing a porous ceramicbody in which such defects can be minimized. In some embodiments, themethods described herein provide porous bodies having appreciable porevolumes, crush strengths, and/or surface areas. In other embodiments,the methods described herein provide porous bodies having a porearchitecture that exhibits enhanced fluid transport properties and highmechanical integrity.

The porous bodies of the present invention can be prepared by firstproviding a precursor mixture, wherein the precursor mixture comprises:(i) milled alpha alumina powder having a particle size of 0.1 to 6microns, (ii) optionally, unmilled alpha alumina powder having aparticle size of 10 to 100 microns (iii) boehmite binder, preferablynanosized, wherein said boehmite functions as a binder of the alphaalumina powders, (iv) burnout material having a particle size of 1-10microns, and (v) optionally, other additives, such as solvents andlubricants. All components of the porous body precursor mixture arehomogeneously mixed.

Another embodiment of the present invention is directed to methods forfabricating a porous body in which the above-described precursor mixtureis formed into a shape, and the formed shape is subjected to a heattreatment process to remove volatiles (e.g., water and burnoutmaterials) and sinter the shape into a porous body. In particularembodiments, the fabrication method comprises: (i) dispersing boehmiteinto water to produce a dispersion of boehmite; (ii) adding a milledalpha alumina powder having a particle size of 0.1 to 6 microns to thedispersion of boehmite, and mixing until a first homogeneous mixture isobtained, wherein said boehmite functions as a binder of the alphaalumina powder; (iii) adding burnout material having a particle size of1-10 microns, and mixing until a second homogeneous mixture is obtained;(iv) forming the second homogeneous mixture to form a shape of saidsecond homogeneous mixture; (v) subjecting the formed shape to a heattreatment step within a temperature in a range of 35-900° C. to removewater and burn out the burnout material to produce a pre-fired porousbody; and (vi) subjecting the pre-fired porous body to a sintering stepat a temperature within a range of 900-2000° C. to produce said porousbody.

In other aspects, the instant disclosure is also directed to the porousbody produced by the above-described method, as well as filters,membranes, catalyst supports, and the like, particularly ethyleneoxidation (i.e., epoxidation) catalysts comprising the porous body(i.e., carrier) described above, along with a catalytic amount ofsilver. In some embodiments, the resulting epoxidation catalyst exhibitsan increased catalyst activity and/or a maintained or improvedselectivity.

The instant disclosure is also directed to a method for the vapor phaseconversion of ethylene to ethylene oxide (EO) by use of theabove-described catalyst. The method includes reacting a reactionmixture comprising ethylene and oxygen in the presence of the ethyleneepoxidation catalyst described above.

DETAILED DESCRIPTION

In one aspect, the instant disclosure is directed to a method forproducing a porous body in which a specially crafted precursor mixtureis formed to a shape and subjected to a heat treatment step to producethe porous body. In particular embodiments, the precursor mixtureincludes at least: (i) milled alpha alumina powder having a particlesize of 0.1 to 6 microns, or more typically, 0.25 to 4 microns, (ii)boehmite powder that functions as a binder of the alpha alumina powders,and (iii) burnout material having a particle size of 1-10 microns. Insome embodiments, the precursor mixture further includes an unmilledalpha alumina powder having a particle size of 10 to 100 microns, whilein other embodiments, the precursor mixture excludes the unmilled alphaalumina. In embodiments, where the unmilled alpha alumina is included,the weight ratio of milled to unmilled alpha alumina powder is generallyin a range of 0.25:1 to about 5:1, preferably 0.5 to 4, and morepreferably, 0.75 to 3. The term “about” generally indicates no more than±10%, 5%, or 1% deviation from a value. The precursor mixture may alsoinclude one or more additives, such as a solvent and/or lubricant. Insome embodiments, the burnout material is selected from at least one ofa polyolefin powder and graphite powder, or from both. In embodimentswhere the burnout material contains both polyolefin and graphite powder,the weight ratio of polyolefin powder to graphite powder is generally ina range of 0.25:1 to about 5:1, preferably in a range of 0.5 to 4, andmore preferably 0.75 to 3. Generally, the boehmite is present in anamount of at least 10% or 25% by weight of total alumina content. Insome embodiments, a silicon-containing substance is substantiallyexcluded from the precursor mixture.

The method for producing the porous body may also be practiced by addingcomponents in at least two steps prior to the heat treatment step. Forexample, in some embodiments, a dispersion of boehmite is firstproduced, i.e., in step (i), by dispersing boehmite particles intowater, which may be neutral water or acidified water. As well known inthe art, boehmite is an aluminum oxide hydroxide material, generallyrecognized as conforming with the formula γ-AlO(OH). For purposes of theinvention, the boehmite particles, as produced in the dispersion, arepreferably nanosized, e.g., up to or less than 200 nm, preferably <100nm, and more preferably <50 nm. The acid employed in the acidified wateris typically a strong mineral acid, such as nitric acid, hydrochloricacid, or sulfuric acid. The acid can be also weak acid, such as, forexample, acetic acid. The acid employed in the acidified water can beadded to neutral water or be dissolved from solid particles, such as,for example, boehmite.

A milled particulate form of alpha-alumina is then added to thedispersion of boehmite in step (ii), wherein the milled form ofalpha-alumina is characterized by an average or median particle size(e.g., D₅₀, the particle size where half of the particle population liesbelow the indicated value) in a range of 0.1 to 6 microns, andpreferably 0.25 to 4 microns. The mixture of boehmite and milledalpha-alumina is mixed until a first homogeneous mixture is obtained.The term “homogeneous,” as used herein, indicates that individualmacroscopic regions of agglomerated particles (i.e., of at least 100 or200 microns) of each substance in the mixture (e.g., boehmite andalpha-alumina) are typically not detectable or present in thehomogeneous mixture, although individual microscopic regions ofagglomerated particles (e.g., less than 100 or 200 microns), may or maynot be present. In the homogeneous mixture, the boehmite functions as abinder of the alpha alumina particles. In some embodiments, thealpha-alumina has a very high purity, i.e., about 95 or 98 wt % or more.In some embodiments, the alpha-alumina is a low sodium alumina or a lowsodium reactive alumina. The term “reactive alumina” as used hereingenerally indicates an alpha-alumina with good sinterability and havinga particle size that is very fine, i.e., generally, of 2 microns orless. Generally, a “low sodium alumina” material contains 0.1% or lesssodium content. Good sinterability is generally derived from a 2 micronor less particle size.

The particle sizes given above can refer to a diameter for the casewhere the particle is spherical or approximately spherical. For caseswhere the particles substantially deviate from a spherical shape, theparticle sizes given above are based on the equivalent diameter of theparticles. As known in the art, the term “equivalent diameter” is usedto express the size of an irregularly-shaped object by expressing thesize of the object in terms of the diameter of a sphere having the samevolume as the irregularly-shaped object.

In some embodiments, step (ii) can include, either simultaneous orsubsequent to adding and mixing the milled alpha alumina powder, addingunmilled alpha-alumina powder having a D₅₀ particle size in a range ofabout 10-100 microns, and mixing until the first homogeneous mixture isobtained. The term “subsequent” indicates that the additional material(e.g., unmilled alpha-alumina) can be included in the same step (ii) orin a succeeding step before the forming and firing steps (iv) to (vi).Typically, the unmilled alpha-alumina has a D₅₀ particle size in a rangeof 10 to 100 microns, and more preferably 25 to 80 microns.

When unmilled alpha-alumina powder is included, the resulting firsthomogeneous mixture contains a homogeneous mixture of boehmite, milledalpha-alumina, and unmilled alpha-alumina. In some embodiments, theweight percentage of milled alpha-alumina is greater than the weightpercentage of unmilled alpha-alumina, by weight of total alumina. Forexample, the milled and unmilled alpha aluminas can be present in aweight ratio (i.e., milled to unmilled alumina) of about, at least, orabove 1.1:1, 1.5:1, 1.8:1, or 2:1 and to up to or less than 1.5:1,1.8:1, 2:1, or 2.5:1. In other embodiments, the weight percentage ofunmilled alpha-alumina is greater than the weight percentage of milledalpha-alumina, by weight of total alumina. For example, the unmilled andmilled alpha aluminas can be present in a weight ratio (i.e., unmilledto milled alumina) of at least or above 1.1:1 or 1.5:1 and to up to orless than 1.8:1, 2:1, or 2.5:1. In other embodiments, the weight ratioof milled to unmilled alpha-alumina is about or at least 0.25:1 or 0.5:1and/or about, up to, or less than 2.5:1 or 3:1. In some embodiments, themilled alpha alumina is the only alumina used in step (ii) or the onlyalumina employed in the method and incorporated into the porous body,i.e., unmilled alpha alumina is excluded from the method. In otherembodiments, the combination of milled and unmilled alpha aluminas isthe only alumina used in step (ii) or the only alumina employed in themethod and incorporated into the porous body.

In some embodiments, the weight percentage of boehmite is about the sameor less than the weight percentage of total alumina. For example, theboehmite may be present in an amount of at least or above 5% or 10%. Insome embodiments, the weight percentage of boehmite is about the same orgreater than the weight percentage of total alumina. For example, theboehmite may be present in an amount of at least or above 25% by weightof total alumina content. The total alumina used in the method in theporous body precursor is typically at least or above 25% or 35% byweight of total weight of solid components incorporated into the porousbody.

After formation of the first homogeneous mixture containing boehmite andalpha-alumina in step (ii), a burnout material having a particle size of1-10 microns is added to and mixed into the first homogeneous mixtureuntil a second homogeneous mixture is obtained, i.e., in step (iii). Thesecond homogeneous mixture preferably consists of free-flowing particlesthat can be subsequently formed to a shape and sintered. The burnoutmaterial, which may also be considered a temporary binder, can be any ofthe burnout materials known in the art, such as granulated polyolefins(e.g., polyethylene or polypropylene), graphite, cellulose, substitutedcelluloses (e.g., methylcellulose, ethylcellulose, andcarboxyethylcellulose), stearates (such as organic stearate esters,e.g., methyl or ethyl stearate), waxes, walnut shell flour, and thelike, which are decomposable at the temperatures employed. The burnoutmaterial is primarily responsible for imparting porosity to the porousbody, and to ensure the preservation of a porous structure during thegreen (i.e., unfired phase) in which the mixture may be shaped intoparticles by molding or extrusion processes. Burnout materials aregenerally substantially or completely removed during firing to producethe finished porous body. In different embodiments, the burnout materialmay have a particle size in a range of about, for example, 1-10 microns,preferably 1-9 microns, and more preferably 1.5-8 microns.

In particular embodiments, the burnout material used in step (iii) is agranulated polyolefin (e.g., polyethylene), or graphite powder, or both.If both the polyolefin and graphite are used, they can have the same ordifferent particle sizes, and they can be added simultaneously orsequentially. For example, in some embodiments, after a granulatedpolyolefin is added and mixed until a second homogeneous mixture isobtained, graphite may be added subsequently, wherein the term“subsequently” or “sequentially” indicates that the additional materialcan be included in the same step (iii) or in a succeeding step beforethe forming and firing steps (iv) to (vi). In some embodiments, thegranulated polyolefin is included in an amount by weight greater thanthe amount by weight of graphite. For example, the weight ratio ofgranulated polyolefin to graphite may be 0.25:1 to about 5:1, andpreferably 0.75 to about 3.5. In other embodiments, the granulatedpolyolefin is included in an amount by weight less than the amount byweight of graphite. For example, the weight ratio of graphite togranulated polyolefin may be 0.25:1 to about 5:1, and preferably 1 toabout 2.5.

In one embodiment, steps (i), (ii), and (iii) are separated andconducted in succession, i.e., the dispersion of boehmite is produced instep (i), followed by production of the first homogeneous mixture instep (ii), followed by production of the second homogenous mixture instep (iii). Steps (i), (ii), and (iii) can be also conducted in reverseor in random order. In another embodiment, steps (i) and (ii) may becombined as a single step, i.e., boehmite and alumina are combined inthe presence of acidified water to form a dispersion of boehmite andalumina, which functions as the first homogeneous mixture. In yetanother embodiment, steps (ii) and (iii) may be combined as a singlestep, i.e., alumina and burnout material are combined during productionof the first homogeneous mixture, which now also functions as the secondhomogeneous mixture. In a further embodiment, steps (i), (ii), and (iii)may be combined as a single step, i.e., boehmite, alumina, and burnoutmaterial are combined in the presence of acidified water to form adispersion of boehmite, alumina, and burnout material, which functionsas the second homogeneous mixture.

In some embodiments, the method further includes (in any step prior toforming and firing the second homogeneous mixture) a binder material insufficient amount. Permanent binders include, for example, inorganicclay-type materials, such as silica and an alkali or alkali earth metalcompound. A convenient binder material which may be incorporated withthe alumina particles comprises boehmite, a stabilized silica sol, andoptionally alkali or alkali earth metal salt. In some embodiments, asilicon-containing substance is substantially or completely excludedfrom the method for producing the porous body. In the case of asilicon-containing substance being substantially excluded from theporous body, a trace amount of silicon derived from impurities in theraw materials used to prepare the porous body may still be present inthe porous body. Such trace amounts are generally no more than 1%, 0.5%,or 0.1% by weight of the porous body.

The precursor mixture, or the second homogeneous mixture formed in step(iii), is then formed into a desired shape by means well known in theart. The forming process can be by extrusion, pressing, pelletizing,molding, casting, etc. After forming, the formed shape is subjected to aheat treatment step in which it is sintered to produce the porous body.The sintering process generally employs a temperature in a range ofabout 900° C. to about 2000° C. The sintering step would alsonecessarily function to remove volatiles, such as water and the burnoutmaterial. However, in some embodiments, a preceding lower temperatureheat treatment (also referred to herein as a “pre-firing step”) isconducted before the sintering step in order to remove such volatiles.The preceding lower temperature heat treatment generally employs atemperature of about 35° C. to about 900° C. Generally, a heating and/orcooling rate within a range of 0.5-100° C./min, preferably 1-20° C./min,or more preferably 2-5° C./min, is used.

In order to properly characterize porous bodies for applications infilters, membranes, or catalyst carriers, pore architecture andconsequently fluid transport-related properties must also be determined.

Among very important parameters in determining the diffusive gastransport through a porous body are tortuosity and constriction.Tortuosity is determined by the ratio of the real length of flow paththrough a porous body to the shortest distance across that porous body(see, for example, B. Ghanbarian et al., Soil Sci. Soc. Am. J., 77,1461-1477 (2013)). Constriction is a function of the area ratio of largepores to small pores. Thus, lowering the values of tortuosity and/orconstriction enhances the diffusive transport through a porous material,i.e., increases the effective diffusivity, which is very important forinstance in catalytic applications.

If there is a pressure drop across the porous body, permeability becomesimportant. Permeability indicates ability of fluids to flow throughporous bodies and can be described by the Darcy's law shown in Equation1, where V is fluid flow velocity, k is permeability, μ is dynamicviscosity of the fluid, ΔP is pressure difference across porous bodywith thickness of Δx:

$\begin{matrix}{V = {\frac{k}{\mu}\frac{\Delta \; P}{\Delta \; x}}} & \left( {{Eq}.\mspace{11mu} 1} \right)\end{matrix}$

Thus higher values of permeability will enhance the pressure-drivenfluid flow across a porous body, which is important in such applicationsas sorption, filtration, or catalysis.

Surprisingly, the aforementioned fluid transport-determining propertiesof porous bodies cannot be found in the literature to characterizeporous architectures, particularly as related to catalyst carriers forepoxidation of olefins. Moreover, there has been no indication in theliterature of the necessary values of tortuosity, constriction orpermeability which provide a pore architecture to a porous body that canachieve enhanced properties, especially in regard to catalystperformance. The present invention provides porous bodies that have apore architecture that has enhanced fluid transport properties and highmechanical integrity.

Unless otherwise specified the following methodology of measurementswere employed in the present application:

In the present invention, water absorption of the porous bodies wasmeasured by placing a 10 g representative sample of a porous body into aflask, which was then evacuated to about 0.1 torr for 5 min.Subsequently, deionized water was aspirated into the evacuated flask tocover the porous bodies while maintaining the pressure at about 0.1torr. The vacuum was released after about 5 minutes to restore ambientpressure, hastening complete penetration of water into the pores.Subsequently, the excess water was drained from the impregnated sample.Water absorption was calculated by dividing total water weight in thepores (i.e., wet mass-dry mass of the sample) by the weight of the drysample at room temperature.

Cumulative intrusion curves and Log differential intrusion curves may beacquired for representative samples of the porous bodies by mercury (Hg)intrusion porosimetry, principles of which are described in Lowell etal., Characterization of Porous Solids and Powders: Surface Area, PoreSize and Density, Springer, 2006. The Hg intrusion pressure may rangebetween, for example, 1.5 and 60,000 psi, which corresponds to poresizes between 140 microns and 3.6 nm. The following Hg parameters may beused for calculations: surface tension of 480 dynes/cm, density of 13.53g/mL, and contact angle of 140°. Pore volumes for the porous bodies maybe measured from the Hg intrusion data, which are consistent with thewater absorption measurements. Additional pore architecture parametersof the porous bodies, such as tortuosity, constriction, andpermeability, may also be calculated from the Hg intrusion data, asdiscussed below.

The tortuosity, ξ, was calculated from Equation 2, where D_(avg) isweighted average pore size, k is permeability, ρ is true materialsdensity, and I_(tot) is total specific intrusion volume (see, forexample, AutoPore V Operator Manual, Micromeritics, 2014):

$\begin{matrix}{\xi = \sqrt{\frac{D_{avg}^{2}}{{4 \cdot 24}{k\left( {1 - {\rho \; I_{tot}}} \right)}}}} & \left( {{Eq}.\mspace{11mu} 2} \right)\end{matrix}$

The constriction, σ, was calculated from Equation 3, where ξ istortuosity and τ is tortuosity factor, calculated from the Carnigiliaequation (see, for example, AutoPore V Operator Manual, Micromeritics,2014):

$\begin{matrix}{\sigma = \frac{\xi}{\tau}} & \left( {{Eq}.\mspace{11mu} 3} \right)\end{matrix}$

The permeability, as defined by the Darcy's law (Eq. 1, above) can becalculated by combining Darcy's and Poiseuille'd equations (see, forexample, Lowell et al., Characterization of Porous Solids and Powders,Springer, 2006). For an arbitrary pore shape factor, f, the permeabilityk is expressed by Equation 4, where τ is tortuosity factor, P ismaterials porosity, and d is pore diameter:

$\begin{matrix}{k = \frac{p^{3}d^{2}}{16f\; {\tau \left( {1 - P} \right)}^{2}}} & \left( {{Eq}.\mspace{11mu} 4} \right)\end{matrix}$

Once tortuosity and pore volumes have been measured, effectivediffusivity can be calculated from Equation 5, where P is materialsporosity, D is diffusivity, D_(eff) is effective diffusivity, and ξ istortuosity [D. W. Green, R. H. Perry, Perry's Engineering Handbook,8^(th) Edition, McGraw-Hill, 2007]

$\begin{matrix}{D_{eff} = \frac{PD}{\xi}} & \left( {{Eq}.\mspace{11mu} 5} \right)\end{matrix}$

In order to calculate absolute values of effective diffusivity, D_(eff),in a porous solid, absolute values of gas diffusivity, D, must be knownper Eq. 5, in addition to the material porosity and tortuosity. However,in order to compare effective diffusivity properties of different poroussolids (e.g., inventive examples of the present invention), it ispossible to calculate relative numbers of effective diffusivitynormalized to a standard material (comparative example of the presentinvention). With the assumption that gas diffusivity, D, is the same inall cases, it requires only knowledge of porosity and tortuosity of theporous materials (see Equation 6).

$\begin{matrix}{\frac{D_{{eff},1}}{D_{{eff},0}} = {\frac{P_{1}}{\xi_{1}}\frac{\xi_{0}}{P_{0}}}} & \left( {{Eq}.\mspace{11mu} 6} \right)\end{matrix}$

Total porosity is defined as the void volume divided by the total volumeof the sample. It can be calculated from mercury porosimetry or waterabsorption, using theoretical density of the carrier material.

The porous body of the present invention typically has a pore volumefrom 0.3 mL/g to 1.2 mL/g. More typically, the porous body of thepresent invention has a pore volume from 0.35 mL/g to 0.9 mL/g. In someembodiments of the present invention, the porous body of the presentinvention has a water absorption from 30 percent to 120 percent, with arange from 35 percent to 90 percent being more typical.

The porous body of the present invention typically has a B.E.T. surfacearea from 0.3 m²/g to 3.0 m²/g. In one embodiment, the porous body ofthe present invention has a surface area from 0.5 m²/g to 1.2 m²/g. Inanother embodiment body of the present invention has a surface areaabove 1.2 m²/g up to, and including, 3.0 m²/g. The B.E.T. surface areadescribed herein can be measured by any suitable method, but is morepreferably obtained by the method described in Brunauer, S., et al., J.Am. Chem. Soc., 60, 309-16 (1938).

The porous body of the present invention can be monomodal, ormultimodal, such as, for example, bimodal. The porous body of thepresent invention has a pore size distribution with at least one mode ofpores in the range from 0.01 micrometers to 100 micrometers. In oneembodiment of the present invention, at least 90 percent of the porevolume of the porous body is attributed to pores having a pore size of20 microns or less. In yet another embodiment of the present invention,at least 85 percent of the pore volume of the porous body is attributedto pores having a size from 1 micron to 6 microns. In yet a furtherembodiment of the present invention, less than 15, preferably less than10, percent of the pore volume of the porous body is attributed to poreshaving a size of less than 1 micron. In still a further embodiment ofthe present application at least 80 percent of the pore volume of theporous body is attributed to pores having a size from 1 micron to 10microns. In a particular aspect of the present invention, there areessentially no pores smaller than 1 micron.

In the case of a multimodal pore size distribution, each pore sizedistribution can be characterized by a single mean pore size (mean porediameter) value. Accordingly, a mean pore size value given for a poresize distribution necessarily corresponds to a range of pore sizes thatresults in the indicated mean pore size value. Any of the exemplary poresizes given above can alternatively be understood to indicate a mean(i.e., average or weighted average) pore size. Each peak pore size canbe considered to be within its own pore size distribution (mode), i.e.,where the pore size concentration on each side of the distribution fallsto approximately zero (in actuality or theoretically). The multimodalpore size distribution can be, for example, bimodal, trimodal, or of ahigher modality. In one embodiment, different pore size distributions,each having a peak pore size, are non-overlapping by being separated bya concentration of pores of approximately zero (i.e., at baseline). Inanother embodiment, different pore size distributions, each having apeak pore size, are overlapping by not being separated by aconcentration of pores of approximately zero.

In one embodiment, the porous body of the present invention may bebimodal having a first set of pores from 0.01 microns to 1 micron and asecond set of pores from greater than 1 micron to 10 microns. In such anembodiment, the first set of pores may constitute less that 15 percentof the total pore volume of the porous body, while the second set ofpores may constitute more than 85 percent of the total pore volume ofthe porous body. In yet another embodiment, the first set of pores mayconstitute less than 10 percent of the total pore volume of the porousbody, while the second set of pores may constitute more than 90 percentof the total pore volume of the porous body.

The porous body of the present invention typically has a total porositythat is from 55 percent to 83 percent. More typically, the porous bodyof the present invention typically has a total porosity that is from 58percent to 78 percent.

The porous body of the present invention typically has an average flatplate crush strength from 10 N to 150 N. More typically, the porous bodyof the present invention typically has an average flat plate crushstrength from 40 N to 105 N. The flat plate crush strength of the porousbodies was measured using a standard test method for single pellet crushstrength of formed catalysts and catalyst carriers, ASTM Standard ASTMD4179.

In some embodiments, the porous body of the present invention can havean attrition value that is less than 40%, preferably less than 25%. Insome embodiments of the present invention, the porous body can haveattrition less that 10%. Attrition measurements of the porous bodieswere performed using a standard test method for attrition and abrasionof catalysts and catalyst carriers, ASTM Standard ASTM D4058.

In some embodiments of the present invention, the porous body of thepresent invention has an initial low alkali metal content. By “lowalkali metal content” it is meant that the porous body contains from2000 ppm or less, typically from 30 ppm to 300 ppm, of alkali metaltherein. Porous bodies containing low alkali metal content can beobtained by adding substantially no alkali metal during the porous bodymanufacturing process. By “substantially no alkali metal” it is meantthat only trace amounts of alkali metal are used during the porous bodymanufacture process as impurities from other constituents of the porousbody. In another embodiment, a porous body having a low alkali metalcontent can be obtained by performing various washing steps to theporous body precursor materials used in forming the porous body. Thewashing steps can include washing in a base, water, or an acid.

In other embodiments of the present invention, the porous body has analkali metal content that is above the value mentioned above for theporous body having substantially no alkali metal content. In such anembodiment the porous body typically contains a measurable level ofsodium on the surface thereof. The concentration of sodium at thesurface of the carrier will vary depending on the level of sodium withinthe different components of the porous body as well as the details ofits calcination. In one embodiment of the present invention, the porousbody has a surface sodium content of from 2 ppm to 150 ppm, relative tothe total mass of the porous body. In another embodiment of the presentinvention, the porous body has a surface sodium content of from 5 ppm to70 ppm, relative to the total mass of the carrier. The sodium contentmentioned above represents that which is found at the surface of thecarrier and that which can be leached, i.e., removed, by, for example,nitric acid (hereafter referred to as acid-leachable sodium).

The quantity of acid leachable sodium present in the porous bodies ofthe present invention can be extracted from the catalyst or carrier with10% nitric acid in deionized water at 100° C. The extraction methodinvolves extracting a 10-gram sample of the catalyst or carrier byboiling it with a 100 ml portion of 10% w nitric acid for 30 minutes (1atm., i.e., 101.3 kPa) and determining in the combined extracts therelevant metals by using a known method, for example atomic absorptionspectroscopy (See, for example, U.S. Pat. No. 5,801,259 and U.S. PatentApplication Publication No. 2014/0100379 A1).

In one embodiment of the present invention, the porous body may have asilica content, as measured as SiO₂, of less than 0.2, preferably lessthan 0.1, weight percent, and a sodium content, as measured as Na₂O, ofless than 0.2 weight percent, preferably less than 0.1 weight percent.In some embodiments, the porous body of the present invention may havean acid leachable sodium content of 40 ppm or less. In yet furtherembodiments of the present invention, the porous body comprises aluminacrystallites having a platelet morphology in a content of less than 20percent by volume. In some embodiments, the alumina crystallites havinga platelet morphology in a content of less than 10 percent by volume arepresent in the porous body of the present invention.

In addition to the above physical properties, the porous body of thepresent invention has a pore architecture that provides at least one ofa tortuosity of 7 or less, a constriction of 4 or less and apermeability of 30 mdarcys or greater. A porous body that has theaforementioned pore architecture has enhanced fluid transport propertiesand high mechanical integrity. In some embodiments, and when used as acarrier for a silver-based epoxidation catalyst, a porous body havingthe aforementioned pore architecture can exhibit improved catalystproperties. Typically, the pore architecture of the porous body of thepresent invention has a tortuosity of 7 or less and/or a constriction of4 or less.

In one embodiment of the present invention, the porous body has a porearchitecture that provides a tortuosity of 7 or less. In anotherembodiment, the porous body of the present invention has a porearchitecture that provides a tortuosity of 6 or less. In yet anotherembodiment, the porous body of the present invention has a porearchitecture that provides a tortuosity of 5 or less. In a furtherembodiment, the porous body of the present invention has a porearchitecture that provides a tortuosity of 3 or less. The lower limit ofthe tortuosity of the porous body of the present invention is 1(theoretical limit). In some embodiments, the tortuosity can be anynumber bounded between 1 and 7.

In one embodiment of the present invention, the porous body has a porearchitecture that provides a constriction of 4 or less. In anotherembodiment, the porous body of the present invention has a porearchitecture that provides a constriction of 3 or less, or even 2 orless. The lower limit of the constriction of the porous body of thepresent invention is 1. In some embodiments, the constriction can be anynumber bounded between 1 and 4.

In another embodiment of the present invention, the porous body has 2-4times improved effective gas diffusivity due to the combination of lowtortuosity and high porosity.

In one embodiment, the porous body of the present invention has a porearchitecture that provides a permeability of 30 mdarcys or greater. Inanother embodiment, the porous body of the present invention has a porearchitecture that provides a permeability of 200 mdarcys or greater.

The porous body can be of any suitable shape or morphology. For example,the carrier can be in the form of particles, chunks, pellets, rings,spheres, three-holes, wagon wheels, cross-partitioned hollow cylinders,and the like, of a size preferably suitable for employment in fixed bedreactors.

In one embodiment, the porous body contains essentially only alumina, oralumina and boehmite components, in the absence of other metals orchemical compounds, except that trace quantities of other metals orcompounds may be present. A trace amount is an amount low enough thatthe trace species does not observably affect functioning or ability ofthe catalyst.

In another embodiment, the porous body may be used as a catalyst carrier(i.e., catalyst support), in which case it typically contains one ormore catalytically active species, typically metals, disposed on or inthe porous body. The one or more catalytically active materials cancatalyze a specific reaction and are well known in the art. In someembodiments, the catalytically active material includes one or moretransition metals from Groups 3-14 of the Periodic Table of Elementsand/or lanthanides. In such applications, one or more promoting species(i.e., species that aide in a specific reaction) can be also disposed onor in the porous body of the present invention. The one or morepromoting species may be, for example, alkali metals, alkaline earthmetals, transition metals, and/or an element from Groups 15-17 of thePeriodic Table of Elements.

In the particular case of the porous body being used as a carrier forsilver-based epoxidation catalysis, the carrier includes silver onand/or in the porous body. Thus, in the method described above,generally after the sintering step, the silver is incorporating on orinto the carrier by means well known in the art, e.g., by impregnationof a silver salt followed by thermal treatment, as well known in theart, as described in, for example, U.S. Pat. Nos. 4,761,394, 4,766,105,4,908,343, 5,057,481, 5,187,140, 5,102,848, 5,011,807, 5,099,041 and5,407,888, all of which are incorporated herein by reference. Theconcentration of silver salt in the solution is typically in the rangefrom about 0.1% by weight to the maximum permitted by the solubility ofthe particular silver salt in the solubilizing agent employed. Moretypically, the concentration of silver salt is from about 0.5% by weightof silver to 45% by weight of silver, and even more typically, fromabout 5% by weight of silver to 35% by weight of silver by weight of thecarrier. The foregoing amounts are typically also the amounts by weightfound in the catalyst after thermal treatment. To be suitable as anethylene epoxidation catalyst, the amount of silver should be acatalytically effective amount for ethylene epoxidation, which may beany of the amounts provided above.

In addition to silver, the silver-based epoxidation catalyst of thepresent invention may also include any one or more promoting species ina promoting amount. The one or more promoting species can beincorporated into the porous body described above either prior to,coincidentally with, or subsequent to the deposition of the silver. Asused herein, a “promoting amount” of a certain component of a catalystrefers to an amount of that component that works effectively to providean improvement in one or more of the catalytic properties of thecatalyst when compared to a catalyst not containing said component.

For example, the silver-based epoxidation catalyst may include apromoting amount of a Group I alkali metal or a mixture of two or moreGroup 1 alkali metals. Suitable Group 1 alkali metal promoters include,for example, lithium, sodium, potassium, rubidium, cesium orcombinations thereof. Cesium is often preferred, with combinations ofcesium with other alkali metals also being preferred. The amount ofalkali metal will typically range from about 10 ppm to about 3000 ppm,more typically from about 15 ppm to about 2000 ppm, more typically fromabout 20 ppm to about 1500 ppm, and even more typically from about 50ppm to about 1000 ppm by weight of the total catalyst, expressed interms of the alkali metal.

The silver-based epoxidation catalyst may also include a promotingamount of a Group 2 alkaline earth metal or a mixture of two or moreGroup 2 alkaline earth metals. Suitable alkaline earth metal promotersinclude, for example, beryllium, magnesium, calcium, strontium, andbarium or combinations thereof. The amounts of alkaline earth metalpromoters are used in similar amounts as the alkali metal promotersdescribed above.

The silver-based epoxidation catalyst may also include a promotingamount of a main group element or a mixture of two or more main groupelements. Suitable main group elements include any of the elements inGroups 13 (boron group) to 17 (halogen group) of the Periodic Table ofthe Elements. In one example, a promoting amount of one or more sulfurcompounds, one or more phosphorus compounds, one or more boron compoundsor combinations thereof can be used.

The silver-based epoxidation catalyst may also include a promotingamount of a transition metal or a mixture of two or more transitionmetals. Suitable transition metals can include, for example, theelements from Groups 3 (scandium group), 4 (titanium group), 5 (vanadiumgroup), 6 (chromium group), 7 (manganese group), 8-10 (iron, cobalt,nickel groups), and 11 (copper group) of the Periodic Table of theElements, as well as combinations thereof. More typically, thetransition metal is an early transition metal selected from Groups 3, 4,5, 6, or 7 of the Periodic Table of Elements, such as, for example,hafnium, yttrium, molybdenum, tungsten, rhenium, chromium, titanium,zirconium, vanadium, tantalum, niobium, or a combination thereof.

In one embodiment of the present invention, the silver-based epoxidationcatalyst includes silver, cesium, and rhenium. In another embodiment ofthe present invention, the silver-based epoxidation catalyst includessilver, cesium, rhenium and one or more species selected from Li, K, W,Zn, Mo, Mn, and S.

The silver-based epoxidation catalyst may also include a promotingamount of a rare earth metal or a mixture of two or more rare earthmetals. The rare earth metals include any of the elements having anatomic number of 57-71, yttrium (Y) and scandium (Sc). Some examples ofthese elements include lanthanum (La), cerium (Ce), and samarium (Sm).

The transition metal or rare earth metal promoters are typically presentin an amount of from about 0.1 micromoles per gram to about 10micromoles per gram, more typically from about 0.2 micromoles per gramto about 5 micromoles per gram, and even more typically from about 0.5micromoles per gram to about 4 micromoles per gram of total catalyst,expressed in terms of the metal. All of the aforementioned promoters,aside from the alkali metals, can be in any suitable form, including,for example, as zerovalent metals or higher valent metal ions.

The silver-based epoxidation catalyst may also include an amount ofrhenium (Re), which is known as a particularly efficacious promoter forethylene epoxidation high selectivity catalysts. The rhenium componentin the catalyst can be in any suitable form, but is more typically oneor more rhenium-containing compounds (e.g., a rhenium oxide) orcomplexes. The rhenium can be present in an amount of, for example,about 0.001 wt. % to about 1 wt. %. More typically, the rhenium ispresent in amounts of, for example, about 0.005 wt. % to about 0.5 wt.%, and even more typically, from about 0.01 wt. % to about 0.05 wt. %based on the weight of the total catalyst including the support,expressed as rhenium metal. All of these promoters, aside from thealkali metals, can be in any suitable form, including, for example, aszerovalent metals or higher valent metal ions.

After impregnation with silver and any promoters, the impregnatedcarrier is removed from the solution and calcined for a time sufficientto reduce the silver component to metallic silver and to remove volatiledecomposition products from the silver-containing support. Thecalcination is typically accomplished by heating the impregnatedcarrier, preferably at a gradual rate, to a temperature in a range ofabout 200° C. to about 600° C., more typically from about 200° C. toabout 500° C., more typically from about 250° C. to about 500° C., andmore typically from about 200° C. or 300° C. to about 450° C., at areaction pressure in a range from about 0.5 to about 35 bar. In general,the higher the temperature, the shorter the required calcination period.A wide range of heating periods have been described in the art for thethermal treatment of impregnated supports. See, for example, U.S. Pat.No. 3,563,914, which indicates heating for less than 300 seconds, andU.S. Pat. No. 3,702,259, which discloses heating from 2 to 8 hours at atemperature of from 100° C. to 375° C. to reduce the silver salt in thecatalyst. A continuous or step-wise heating program may be used for thispurpose. During calcination, the impregnated support is typicallyexposed to a gas atmosphere comprising an inert gas, such as nitrogen.The inert gas may also include a reducing agent.

In another embodiment, the porous body described above can also be usedas a filter in which liquid or gas molecules can diffuse through thepores of the porous body described above. In such an application, theporous body can be placed along any portion of a liquid or gas streamflow. In yet another embodiment of the present invention, the porousbody described above can be used as a membrane.

In another aspect, the invention is directed to a method for the vaporphase production of ethylene oxide by conversion of ethylene to ethyleneoxide in the presence of oxygen by use of the silver-based epoxidationcatalyst described above. Generally, the ethylene oxide productionprocess is conducted by continuously contacting an oxygen-containing gaswith ethylene in the presence of the catalyst at a temperature in therange from about 180° C. to about 330° C., more typically from about200° C. to about 325° C., and more typically from about 225° C. to about270° C., at a pressure which may vary from about atmospheric pressure toabout 30 atmospheres depending on the mass velocity and productivitydesired. Pressures in the range of from about atmospheric to about 500psi are generally employed. Higher pressures may, however, be employedwithin the scope of the invention. Residence times in large-scalereactors are generally on the order of about 0.1 to about 5 seconds. Atypical process for the oxidation of ethylene to ethylene oxidecomprises the vapor phase oxidation of ethylene with molecular oxygen inthe presence of the inventive catalyst in a fixed bed, tubular reactor.Conventional commercial fixed bed ethylene oxide reactors are typicallyin the form of a plurality of parallel elongated tubes (in a suitableshell). In one embodiment, the tubes are approximately 0.7 to 2.7 inchesO.D. and 0.5 to 2.5 inches I.D. and 15-45 feet long filled withcatalyst.

In some embodiments, the silver-based epoxidation catalyst describedabove exhibits a high level of selectivity in the oxidation of ethylenewith molecular oxygen to ethylene oxide. For example, a selectivityvalue of at least about 83 mol % up to about 93 mol % may be achieved.In some embodiments, the selectivity is from about 87 mol % to about 93mole %. The conditions for carrying out such an oxidation reaction inthe presence of the silver-based epoxidation catalyst described abovebroadly comprise those described in the prior art. This applies, forexample, to suitable temperatures, pressures, residence times, diluentmaterials (e.g., nitrogen, carbon dioxide, steam, argon, and methane),the presence or absence of moderating agents to control the catalyticaction (e.g., 1, 2-dichloroethane, vinyl chloride or ethyl chloride),the desirability of employing recycle operations or applying successiveconversion in different reactors to increase the yields of ethyleneoxide, and any other special conditions which may be selected inprocesses for preparing ethylene oxide.

In the production of ethylene oxide, reactant feed mixtures typicallycontain from about 0.5 to about 45% ethylene and from about 3 to about15% oxygen, with the balance comprising comparatively inert materialsincluding such substances as nitrogen, carbon dioxide, methane, ethane,argon and the like. Only a portion of the ethylene is typically reactedper pass over the catalyst. After separation of the desired ethyleneoxide product and removal of an appropriate purge stream and carbondioxide to prevent uncontrolled build up of inert products and/orby-products, unreacted materials are typically returned to the oxidationreactor.

Examples have been set forth below for the purpose of furtherillustrating the invention. The scope of this invention is not to be inany way limited by the examples set forth herein.

Examples Preparation and Characterization of Alumina-Based PorousSupports

Typical compositions of the precursor mixtures of the porous bodies ofthe present invention are shown in Table 1. Porous bodies of the presentinvention were typically prepared under constant stirring by (i)dispersing boehmite in water; (ii) adding milled and/or unmilled alphaalumina powder; (iii) adding burn-out 1, if any; (iv) adding burn-out 2if any; and (v) adding lubricant. Particular quantities and types of theindividual constituents of each precursor mixture of the presentinvention are shown in Table 1. Subsequently, the mixture was extrudedusing 2″ Bonnot extruder with a single die to produce extrudate in theshape of hollow cylinders. The extrudates were cut into equal-lengthpieces and then dried under a heat lamp for 1 hr. Subsequently, the cutand dried extrudates were moved to a furnace and subjected to thefollowing heat treatments: (i) pyrolysis of the burn-out was performedin flowing air at 800° C. for 16 hrs with average heating rate of 23°C./hr; followed by (ii) sintering at 1250-1550° C. for 12 hrs with aheating and cooling rates of 2.0° C./min.

The following compositions were produced and analyzed, and the resultsprovided in the following tables:

TABLE 1 Ranges of compositions of porous body precursors Precursor forUnmilled Alpha Milled Alpha Solvents and Porous Body Alumina PowderAlumina Powder Boehmite Lubricants Total Burn-out 1 Burn-out 2 No. (g)(g) (g) (g) (g) (g) PB1 250-500 500-700 100-300 500-700 250-500  0-200(Inventive Example) PB2  0-250 500-700 200-400 500-800 300-600 150-350(Inventive Example) PB3 200-450 500-700 150-350 600-950 350-650 100-300(Inventive Example) PB4 600-900 500-700 100-250 500-700 250-450 —(Inventive Example) PB5 500-700 700-900 100-250 500-700 250-450 —(Inventive Example) PB6 1,500 — 100-250 700-900 — — (Inventive Example)PB7  0-150 500-700 150-350 500-700 350-550 100-300 (Inventive Example)PB8 Comparative Example PB8 was made using a different methodology,(Comparative not of the present invention Example)

TABLE 2 Ranges of properties for different porous bodies BET AverageEffective Porous Body Pore Surface Crush Diffusivity Composition VolumeArea Strength Tortuosity Constriction Permeability normalized No. (mL/g)(m²/g) (N) (—) (—) (mdarcy) to PB8 PB1 0.55-0.62 0.9-1.0 51-77 3.8 2.6220 2.38 (Inventive Example) PB2 0.66-0.84 0.6-1.2 37-60 3.3 2.3 88 2.88(Inventive Example) PB3 0.58-0.82 0.7-0.9 10-62 3.1 2.2 241 3.04(Inventive Example) PB4 0.49-0.53 0.9-1.2 46-60 2.4 1.6 397 3.60(Inventive Example) PB5 0.43-0.52 0.6-1.1  51-105 2.5 1.7 678 3.49(Inventive Example) PB6 0.35-0.45 0.7-0.8 43-74 3.1 2.1 731 2.74(Inventive Example) PB7 0.60-0.90 1.0-2.2 64-94 4.3 3.0 37 2.21(Inventive Example) PB8 0.35-0.55 0.4-1.0 50-80 8.3 5.3 15 1.00(Comparative Example)

While there have been shown and described what are presently believed tobe the preferred embodiments of the present invention, those skilled inthe art will realize that other and further embodiments can be madewithout departing from the spirit and scope of the invention describedin this application, and this application includes all suchmodifications that are within the intended scope of the claims set forthherein.

What is claimed is:
 1. A precursor mixture for producing a porous body, wherein the precursor mixture comprises: (i) at least one milled alpha alumina powder having a particle size of 0.1 to 6 microns, (ii) boehmite powder that functions as a binder of the alpha alumina powders, and (iii) at least one burnout material having a particle size of 1-10 microns.
 2. The precursor mixture of claim 1, further comprising unmilled alpha alumina powder having a particle size of 10 to 100 microns.
 3. The precursor mixture of claim 2, wherein the weight ratio of milled to unmilled alpha alumina powder is in a range of 0.25:1 to about 5:1.
 4. The precursor mixture of claim 1, wherein unmilled alpha alumina powder is excluded from the precursor mixture.
 5. The precursor mixture of claim 1, further comprising an additive selected from solvents and lubricants.
 6. The precursor mixture of claim 1, wherein said burnout material is selected from at least one of a polyolefin powder and graphite powder.
 7. The precursor mixture of claim 1, wherein said burnout material is selected from both a polyolefin powder and graphite powder.
 8. The precursor mixture of claim 7, wherein the weight ratio of polyolefin powder to graphite powder is in a range of 0.25:1 to about 5:1.
 9. The precursor mixture of claim 1, wherein the boehmite is present in an amount of at least 10% by weight of total alumina content.
 10. The precursor mixture of claim 1, wherein the boehmite is present in an amount of at least 25% by weight of total alumina content.
 11. The precursor mixture of claim 1, wherein the boehmite is nano-sized powder with dispersed particle size <100 nm.
 12. The precursor mixture of claim 1, wherein said milled alpha alumina powder has a particle/crystallite size of 0.25-4 microns.
 13. The precursor mixture of claim 1, wherein a silicon-containing substance is substantially excluded from the precursor mixture.
 14. The precursor mixture of claim 1, wherein a sodium-containing substance is substantially excluded from the precursor mixture.
 15. A method for producing a porous body, the method comprising: providing a precursor mixture comprising (i) milled alpha alumina powder having a particle size of 0.1 to 6 microns, (ii) boehmite powder that functions as a binder of the alpha alumina powders, and (iii) burnout material having a particle size of 1-10 microns; forming a predetermined shape; and subjecting the shape to a heat treatment step in which the shape is sintered to produce the porous body.
 16. The method of claim 15, further comprising unmilled alpha alumina powder having a particle size of 10 to 100 microns in said precursor mixture.
 17. The method of claim 16, wherein the weight ratio of milled to unmilled alpha alumina powder is in a range of 0.25:1 to about 5:1.
 18. The method of claim 15, wherein unmilled alpha alumina powder is excluded from the precursor mixture.
 19. The method of claim 15, wherein the method comprises: (i) dispersing boehmite into water to produce a dispersion of boehmite; (ii) adding a milled alpha alumina powder having a particle size of 0.1 to 6 microns to the dispersion of boehmite, and mixing until a first homogeneous mixture is obtained, wherein said boehmite functions as a binder of the alpha alumina powder; (iii) adding burnout materials having a particle size of 1-10 microns, and mixing until a second homogeneous mixture is obtained; (iv) forming the second homogeneous mixture to form a shape of said second homogeneous mixture; and (v) subjecting the formed shape to a heat treatment step in which the formed shape is sintered to produce the porous body.
 20. The method of claim 15, wherein said heat treatment step comprises: (a) subjecting the formed shape to a heat treatment step within a temperature in a range of 35-900° C. to remove water and burn out the burnout material to produce a pre-fired porous body; and (b) subjecting the pre-fired porous body to a sintering step at a temperature within a range of 900-2000° C. to produce said porous body.
 21. The method of claim 15, wherein said porous body possesses at least one of a water absorption of at least 30%, a crush strength of at least 40 N, and a BET surface area of at least 0.3 m²/g.
 22. The method of claim 15, wherein said porous body possesses a pore architecture that provides at least one of a tortuosity of 7 or less, a constriction of 4 or less, and a permeability of 30 mdarcys or greater.
 23. The method of claim 15, wherein said burnout material is selected from a polyolefin powder and graphite powder.
 24. The method of claim 19, wherein said burnout material comprises a polyolefin powder, and said step (iii) comprises adding said polyolefin powder having a particle size of 1-10 microns, and mixing until a second homogeneous mixture is obtained.
 25. The method of claim 24, wherein said step (iii) further comprises, either simultaneous or subsequent to adding and mixing the polyolefin powder, adding graphite powder as an additional burnout material, and mixing until said second homogeneous mixture is obtained, which includes the graphite powder.
 26. The method of claim 25, wherein the weight ratio of polyolefin powder to graphite powder is in a range of 0.25:1 to about 5:1.
 27. The method of claim 25, wherein said graphite powder has a particle size of 3-10 microns.
 28. The method of claim 19, wherein said step (ii) includes, either simultaneous or subsequent to adding and mixing the milled alpha alumina powder, adding unmilled alpha alumina powder having a particle size in a range of 10-100 microns, and mixing until said first homogeneous mixture is obtained.
 29. The method of claim 28, wherein the weight ratio of milled to unmilled alpha alumina powder is in a range of 0.25:1 to about 5:1.
 30. The method of claim 15, wherein the boehmite is present in an amount of at least 10% by weight of total alumina content.
 31. The method of claim 15, wherein the boehmite is present in an amount of at least 25% by weight of total alumina content.
 32. The method of claim 15, wherein the boehmite is nano-sized powder with dispersed particle size <100 nm.
 33. The method of claim 15, wherein unmilled alpha alumina powder is excluded from the method to produce the porous body.
 34. The method of claim 15, wherein a silicon-containing substance is substantially excluded from the method to produce the porous body.
 35. The method of claim 15, wherein a sodium-containing substance is substantially excluded from the precursor mixture.
 36. The method of claim 15, wherein, after said heat treatment step to form a porous body, said method further comprises depositing silver on and/or in said porous body.
 37. The method of claim 15, wherein said milled alpha alumina powder has a particle size of 0.25-4 microns.
 38. The method of claim 15, wherein the precursor mixture is formed by one of extrusion or pressing. 